It is common in the art to utilize an antenna array comprised of a plurality of antenna elements in order to illuminate a selected area with a signal or signals. Often such an array is used in combination with beam forming techniques, such as phase shifting the signal associated with particular antenna elements of the array, such that the signals from the excited elements combine to form a desired beam, or radiation pattern, having a predetermined shape and/or direction.
For example beam forming matrices coupled to an antenna array, such as a phased array panel antenna, have been used in providing multiple antenna beams. One such solution utilizes a four by four Butler matrix, having four inputs to accept radio frequency signals and four outputs each of which is coupled to an antenna element or column of elements of a panel phase array antenna, to provide four antenna beams, such as four 30xc2x0 directional antenna beams. Each of the antenna beams of the above phased array is associated with a particular input of the beam forming matrix such that a signal appearing at a first input of the beam forming matrix will radiate in a first antenna beam. This is accomplished by the input signal being provided to each of the four antenna elements, coupled to the outputs of the beam forming matrix, as signal components having a proper phase and/or power relation to one another. Likewise, a signal appearing at a second input of the beam forming matrix will radiate in a second antenna beam. As above, this is accomplished by the input signal being provided to each of the four antenna elements as signal components having a proper phase and/or power relation to one another which is different than the phase and/or power relation as between the signal components of the first beam. Accordingly, the beam forming matrix provides a spatial transform of the signal provided at a single input of the beam forming matrix.
A system such as the multiple beam system described above may be utilized to communicate signals in areas other than those of each individual antenna beam. For example, in the above described embodiment providing four 30xc2x0 directional antenna beams, a signal might be simulcast from a plurality of the antenna beams to thereby communicate the signal in an area different than that associated with a single antenna beam, e.g., two antenna beams to synthesize a 60xc2x0 beam or four of the antenna beams to synthesize a 120xc2x0 beam. However, it should be appreciated that each of the antenna beams in the above described simulcast has a common phase center, i.e., each antenna beam sourced from the aforementioned beam forming matrix using the same antenna elements results in each such antenna beam having a common point of origin or phase center. Therefore, in order to avoid undesired destructive combining of the signal simulcast, it is desirable to present the signal to be simulcast to the beam forming inputs with a zero relative phase distribution, i.e., in the four input Butler matrix example discussed above a relative phase distribution of a signal to be simulcast on each of the four antenna beams would preferably be 0xc2x0, 0xc2x0, 0xc2x0, 0xc2x0, or each simulcast signal in phase at their respective beam forming matrix inputs.
Moreover, where a zero relative phase distribution is present at the beam forming inputs, beam shaping or additional beam forming control may be predictably accomplished through the use of signal amplitude or power level control. For example, to provide a desired radiation pattern a signal may be simulcast on several antenna beams with a different amplitude (whether a signal of greater or lesser magnitude) as provided to one or more of the beam forming inputs. Such systems may be utilized to provide synthesized antenna beam patterns substantially more complex than the aforementioned composite antenna beam patterns otherwise associated with a simulcast technique.
However, disposing signal attenuators in the antenna beam signal paths subsequent to amplification of the signal for transmission will generally result in dissipation of a portion of the power component of the signal. Achieving the power levels often required for proper signal communication, such as the power levels required of a cellular or PCS base transceiver station (BTS), is typically a very expensive proposition. Accordingly, it is not generally desired to utilize a system structure in which a portion of this power is dissipated or otherwise not actually utilized in the transmission of the signal.
One solution to the problem of not fully utilizing signal power for transmission of the signal might be to place the signal attenuation circuitry in the antenna beam signal paths prior to amplification of the signal for transmission. Accordingly, only a relatively small amount of signal power may be dissipated to provide a signal attenuated to a level such that, when the amplifier stage gain is added thereto, a desired relative amplitude is provided to the corresponding beam forming input. However, this solution presents its own set of problems to the communication system. Specifically, such an embodiment would typically require the removal of the amplifiers from an existing BTS system configuration in order to allow disposition of controllable attenuators in the individual signal paths prior to amplification. However, because amplification of the signals to be transmitted is often a critical function, the amplifiers may be alarmed or otherwise monitored for proper operation. This may cause substantial implementation problems when attempting to provide an applique to retrofit existing BTS systems with a smart antenna providing complex radiation pattern synthesis.
Accordingly, a need exists in the art for a system and method adapted to provide controlled relative power levels with respect to simulcast signals which do not result in undesired power dissipation or other substantial waste.
A further need exists in the art for a system and method providing controlled relative power levels with respect to simulcast signals while minimizing the impact on existing system implementations.
A still further need exists in the art for a system and method providing controlled relative power levels of corresponding signals having a predetermined relative phase relationship without substantially affecting such relative phase relationship.
The present invention is directed to a system and method in which signal power steering circuitry is utilized to provide controlled relative power levels with respect to a plurality of corresponding signals, such as signals to be simulcast in synthesizing a desired antenna beam. A preferred embodiment of the present invention utilizes a multiple stage circuit adapted to shift or steer signal power from a stage input between stage outputs.
For example, a most preferred embodiment of the present invention utilizes a matrix of back-to-back hybrid combiners, such as 90xc2x0 hybrid combiners, to provide a power steering circuit. The back-to-back combiner arrangement of this embodiment provides a first hybrid combiner having a first output coupled to a first input of a second hybrid combiner and having a second output coupled to a second input of the second hybrid combiner. Preferably the back-to-back hybrid combiners have a controllable phase shifter in at least one link there between to allow control of signal power levels at the outputs of the second hybrid combiner of the back-to-back pair by selectively directing input power to the outputs of the hybrid combiner pair.
By coupling a plurality of such back-to-back hybrid combiner pairs into a matrix, stages of power steering may be accomplished according to the present invention. For example, where a four input beam forming matrix is utilized in providing four directional antenna beams, a two stage back-to-back hybrid combiner matrix may be utilized according to the present invention to provide desired relative power level distribution of a signal to each of the four beam forming inputs. Specifically, a first stage of the matrix may provide coarse power steering, such as between a first and second half of the beam forming inputs, and a second stage of the matrix may provide fine power steering, such as between individual beam forming inputs.
The preferred embodiment of the present invention is adapted to maintain, or otherwise achieve, a desired relative phase relationship of the signals provided to the beam forming inputs. For example, according to a most preferred embodiment of the present invention a zero phase relationship is maintained at the beam forming inputs. Accordingly, a preferred embodiment of the present invention includes phase control circuitry, such as disposed between one or more of the power steering stages, suitable for use in maintaining and/or providing a desired relative phase relationship. A most preferred embodiment of the present invention includes a controllable phase shifter in at least one signal path of a power steering stage to thereby control phase drift between signal paths of that particular power steering stage.
An advantage of the present invention is provided in that the corresponding signals relative power levels are provided through steering of the power to the appropriate signal path rather than through dissipation or other sinking of the signal power.
A further advantage of the present invention is that a desired relative phase relationship between the corresponding signals may be maintained.
A still further advantage of the present invention is provided in that preferred embodiment of the present invention may be implemented as an applique and, therefore, minimize the impact on an existing system implementation.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.